1. Field of the Invention
The invention relates generally to apparatus, systems, and methods for separating and collecting particulate matter from a fluid. More particularly, the invention relates to a wetted wall cyclone and method of using the same for separating and collecting particular matter on a liquid layer or film. Still more particularly, the invention relates to a wetted wall cyclone and method of using the same for bioaerosol collection and concentration in ambient conditions.
2. Background of the Invention
A cyclone separator is a mechanical device employed to remove and collect particulate matter or solids from a gas, typically air, by the use of centrifugal force. The gaseous suspension containing the fine particulate matter, often referred to as an “aerosol,” is tangentially flowed into the inlet of a cyclone body, resulting in a vortex of spinning airflow within the cyclone body. As the aerosol enters the cyclone, it is accelerated to a speed sufficient to cause entrained particles having sufficient inertia to move radially outward under centrifugal forces until they strike the inner wall of the cyclone body.
In a wetted wall cyclone, the particulate matter moving radially outward is collected on a liquid film or layer, also known as a “collection fluid” or “collection liquid”, that is formed on at least a portion of the inner surface of the cyclone wall. The liquid film is created by injecting the liquid into the cyclone body. When injected into the cyclone body, the liquid may be atomized into droplets, which are then deposited on the inner wall of the cyclone to form the liquid film. The liquid may be continuously injected or applied at periodic intervals to wash the inner surface of the cyclone wall. Shear forces caused by the cyclonic bulk airflow, which may be aided by the force of gravity, cause the liquid layer on the inner surface of the cyclone wall, as well as the particulate matter entrained therein, to move axially along the inner surface of the cyclone wall as a film, as droplets, or as rivulets toward a skimmer positioned downstream of the cyclone body. In wetted wall cyclone separators using water as the injected collection fluid, the suspension of water and entrained particulate matter is often referred to as a “hydrosol”.
The liquid film, droplets, or rivulets on the inner surface of the cyclone wall including the entrained particulate matter are separated from the bulk airflow by a skimmer from which the liquid film and particles entrained therein are aspirated from the cyclone body. The processed or “cleansed” air (i.e., the air remaining after the particulate matter has been separated) exits the cyclone body and may be exhausted to the environment or subject to further separation. In this manner, at least a portion of the particulate matter in the bulk airflow is separated and collected in a more concentrated form that may be passed along for further processing or analysis. The concentration of the particulate matter separated from the bulk airflow can be increased by several orders of magnitude by this general process.
Wetted wall cyclone separators are used for a variety of separating and sampling purposes. For instance, wetted wall cyclones may be used as part of a bioaerosol detection system in which airborne bioaerosol particles are separated and collected in a concentrated form that can be further analyzed to assess the characteristics of the bioaerosol particles.
The effectiveness or ability of the cyclone separator to separate and collect such particulate matter is often measured by the aerosol-to-hydrosol collection efficiency which is calculated by dividing the amount of particles of a given size that leave the cyclone separator in the hydrosol exhaust stream by the amount of particles of that same size that enter the cyclone in the bulk airflow or aerosol state.
In most conventional wetted wall cyclones, the liquid skimmer is connected to the cyclone body at a location where the cyclone body has an expanded or increased radius section. In such a diverging flow region, the cyclonic airflow tends to decelerate in the axial direction. As a result, the hydrosol liquid flowing along the inner wall of the cyclone body proximal the skimmer may collect and buildup in a relatively stagnant ring-shaped torus. Some of the hydrosol contained within such a torus may be undesirably swept up and entrained in the cyclonic airflow, and exit the cyclone body along with such separated airflow, thereby bypassing the skimmer and associated aspiration. This phenomenon, often referred to as “liquid carryover”, degrades the cyclone's separation and collection capabilities. In particular, liquid carryover can significantly decrease the aerosol-to-hydrosol collection efficiency. For instance, Battelle Memorial Institute, Columbus, Ohio developed a wetted wall cyclone that was designed to operate at an air flow rate of 780 L/min and an effluent liquid flow rate of about 1.5 mL/min. The aerosol-to-hydrosol collection efficiency for particles in the size range of 1.5 to 6.5 μm aerodynamic diameter (AD) is about 60%; however, the unit frequently exhibits water carryover which significantly reduces the aerosol-to-hydrosol efficiency.
New systems are being developed for near-real-time analyses of bioaerosols, which can provide detection and identification of hazardous bioaerosol particles. These systems typically require samples in the hydrosol form with an equivalent liquid flow rate on the order of one-hundred μL/min. Such systems may be employed to sample air from occupied environments at room temperature or from ambient environments, where both relatively high and low temperatures may be experienced.
The effectiveness and efficiency of a wetted wall cyclone operated in a sub-freezing environment may be significantly reduced if the injected liquid and/or the hydrosol begin to solidify or freeze. It may be particularly desirable, for these applications, to control the temperature of the cyclone body, injected liquid, hydrosol, or combinations thereof, to prevent solidification of the wetting liquid. Thus, in such environments the skimmer must also be designed to ensure effective aspiration of the liquid including operation in sub-freezing conditions. As another example, in cases where the wetted wall cyclone is employed to sample bioaerosols, it is preferred that the collected aerosol particles contained in the liquid be preserved for further analysis and study. The preservation of biological materials may necessitate a particular temperature range within the cyclone. Many conventional wetted wall cyclones do not include any means or mechanism to control the temperature of the cyclone body, injected fluid, or hydrosol. In addition, although the Battelle cyclone previously discussed employs an electric heating element to control the temperature of the cyclone body, its effectiveness drops off significantly in environments having an ambient temperature below about −10° C. even when heated with a relatively large 350 watts of electrical power. Still further, most conventional heated wetted wall cyclones employ a single heater to control the temperature of the cyclone body. However, the air flow patterns within the cyclone body result in variations in local turbulent heat transfer coefficients, which can result in temperature gradients along the cyclone body. Moreover, in heated wetted wall cyclones employing a single heat source, undesirable hot spots and/or cold spots often develop on the cyclone body. Such hot or cold spots may damage biological materials, and/or may cause partial solidification of the collection liquid in certain regions of the cyclone body. Furthermore, relying on the use of a single large heater in sub-freezing environments may require undesirably high power consumption.
For relatively hot-dry ambient environments, collection fluid evaporation has conventionally been addressed by simply increasing the flow rate of the injected collection fluid. However, this may be problematic for samplers positioned at remote locations, where additional collection fluid is not readily available, and thus, the use of minimal amounts of collection fluid is desirable. Batch-type aerosol samplers have been reported where a known amount of liquid is added to the collector prior to use, the collector is operated for the desired period of time during which makeup liquid is added to compensate for evaporation losses, and the liquid is recovered for analysis. However, such batch systems do not enable near-real-time detection capabilities, and further, still require makeup liquid during operation in hot-dry environments.
Accordingly, there remains a need in the art for sampling systems and methods for operation thereof enabling operation in ambient conditions, including relatively hot-dry as well as sub-freezing environments. Particularly well received would be a wetted wall cyclone separator and method operable with minimal liquid consumption, minimal power consumption, and/or variable temperature control of select areas of the cyclone body and concomitantly offering the potential for reduced liquid carryover and/or improved efficiency.